The invention relates to a level sensing apparatus and method for a fluid transfer mechanism transferring aliquots of conductive sample fluids from a supply to reaction vessels. More particularly, this invention relates to an apparatus and method for determining when the bottom tip of a fluid aspirating probe touches or makes contact with the upper surface of a sample fluid contained in a supply cavity.
Fluid transfer mechanisms are known which aspirate a precise aliquot or portion of a sample fluid from a supply in one location and dispense the aspirated aliquot in a reaction vessel located at another location. Such a device disclosed in the referenced application FLUID TRANSFER MECHANISM, V. Drbal, Ser. No. 115,691, filed Jan. 28, 1980, has the capability of transferring the desired fluid aliquot at a high rate of speed and with very precise positioning of a fluid aspirating and despensing probe in the vertical and horizontal directions.
It is important in such fluid transfer mechanisms that only the tip of the fluid aspirating probe be in contact with the sample fluid to be aspirated to avoid the sample probe carrying undesired quantities of the sample fluid on the exterior thereof. The reactions which are performed with the sample fluids usually concern or are performed to determine the condition of a patient's health and as such the quantity of the sample fluids which are used in the reactions must be precisely and accurately controlled. Undesired excess quantities on the exterior of the probe may affect reactions and provide misleading data concerning the condition of the patient's health.
The use of capacitive sensing in the detection of fluid levels is known, but in an apparatus where stray capacitance is of an order as great or greater than the capacitance involved in the sensing operation problems arise. This will occur where the liquid sample is extremely small as contemplated by the chemical analyzer mentioned. For instance the operator may upset data or sensing information by merely having his hand in the vicinity of the fluid transfer device.